The invention relates to bipolar output circuits compatible with TTL-type circuits, and more particularly to high speed output circuits without Schottky devices (the latters not being usable in some technological implementations).
The TTL compatible output circuits have to supply either signals higher than 2.4 V, or signals lower than 0.4 V. In the latter case, the output circuit has to stand a current of about 16-24 mA applied to its output terminal.
FIG. 1 shows a conventional bipolar output circuit. It is fed by a supply source, a first terminal 1 of which supplies a positive voltage VCC and the second terminal 2 is grounded. It comprises an input terminal 3 and an output terminal 4. A first NPN output transistor T1 has its collector connected to terminal 1 through a resistor RA and its emitter connected to the output terminal through a diode 5. A second NPN output transistor T2 has its collector connected to the output terminal and its emitter connected to the terminal 2 of the supply source. The collector of an NPN control transistor T3 is connected on the one hand to the base of transistor T1 and on the other hand through a resistor RB to the terminal 1 of the supply source. The emitter of transistor T3 is connected to the base of transistor T2 and is connected, through a resistor RC, to terminal 2 of the supply source. Transistor T3 has its base connected on the one hand to terminal 1 through a resistor RD and on the other hand to input terminal 3 through a diode 6.
Two voltage levels are liable to be applied to the input terminal: a low voltage level (low state) and a high voltage level (high state). When the input is at a low level, diode 6 is forward biased. Transistor T3, as well as transistor T2, are blocked. Transistor T1 is conductive and the output is at a high level. When the input is at a high level, transistors T3 and T2 are conductive and transistor T1 is blocked. The output is at a low level.
The circuit of FIG. 1 satisfactorily operates if the transistors and the diodes represented are of the Schottky type. The switching speed is then very high (about a few nanoseconds). However if, as illustrated, the transistors and diodes are of the conventional bipolar type, two phenomena limit the switching speed.
On the one hand, if some transistors operate in a saturation mode in the circuit, their switching off speed state is reduced.
On the other hand, during the switching on phase of a transistor, the switching speed is reduced due to Miller effect.
When a transistor is in a saturation mode, the fact that current i.sub.B in the base is relatively high, the base voltage being higher than the collector voltage, causes charges to be stored in the transistor base (if I.sub.C is the current in the collector and .beta. is the transistor gain, I.sub.C /i.sub.B &lt;&lt;.beta. in the saturation mode). Thus, in order to avoid the saturation mode, the current in the base is limited when the transistor is conductive, or the voltage drop between the base and the collector is determined so that it is strictly lower than a threshold voltage V.sub.S which is in practice equal to about the voltage V.sub.BE existing between the base and the emitter of a transistor in the conductive state.
In order to limit the Miller effect at the switching on, one will limit the voltage gain of the transistors for which the charge of the base-collector capacitor is critical, this charge originating from the current arriving on the base.
In the circuit of FIG. 1, when output 4 is at a low state, each of the two transistors T2 and T3 presents a low voltage drop between collector and emitter. The voltage difference between the base and the collector is roughly equal to the voltage drop V.sub.BE between the base and the emitter, which conventionally is about 0.7 V. The voltage drop between the base and the collector is roughly equal to the threshold voltage V.sub.S. Transistors T2 and T3 operate in the saturation mode.
Moreover, the charging of the base-collector capacitor of transistor T3 at the switching on is slow due to the presence of resistor RD. Moreover, the voltage gain of transistor T3 is high due to the presence of resistor RB. Therefore, transistor T3 exhibits an important Miller effect at the switching on.
In the prior art, in order to solve this problem, it has been tried to provide for output circuits without high speed switching Schottly devices. For this purpose, efforts have been mainly mode to prevent the pull down transistor (T2) from operating in the saturation mode. This brings a first improvement. However, when considering all the known circuits, it appears they all include, in control stages, some transistors operating in the saturation mode when they are in a conductive state or some transistors having a high voltage gain, for which the charge of the base-collector capacitor is critical and which therefore present an important Miller effect.